Different applications in communications systems mostly operate with different data rates. But the underlying transmission channels, however, mostly offer, because they are embedded into certain transmission formats, only a fixed data transmission rate or a raw data transmission rate or only a discrete set of such data rates. Thus, it will be necessary, in general, to match the data rates to each other at the corresponding interface. This is described below using an example from the UMTS standardization.
At present, work is in progress on standardizing what is known as the UMTS (Universal Mobile Telecommunications System) mobile radio standard for third-generation mobile radio devices. In accordance with the known current state of UMTS standardization there is provision for subjecting the data to be transferred via a high frequency channel to channel coding, in which case convolutional codes are particularly used. The data to be transmitted is coded redundantly by channel coding which makes a more reliable retrieval of the transmitted data possible on the receiver side. The code used in each case for channel coding is characterized by its code rate r=k/n, where k is the number of data bits or message bits to be transmitted and n is the number of the bits present after encoding. As a rule, the smaller the code rate the more powerful the coding. A problem associated with coding, however, is that the data rate is reduced by a factor of r.
Rate matching is performed in the transmitter to adapt the data rate of the coded data stream to the relevant possible transmission rate with bits being either removed from the data stream in accordance with a specific pattern or duplicated in the data stream. The removal of bits is the called “puncturing” and the duplication is called “repetition”.
According to the current status of UMTS standardization, it is proposed for rate matching that an algorithm be used which performs puncturing with an almost regular puncturing pattern, with the bits to be punctured being distributed equidistantly over the coded data block to be punctured in each case.
In addition, it is known that for convolutional coding the bit error rate (BER) decreases at the edge of a correspondingly coded data block. It is also known that the bit error rate within a data block can be changed locally by regularly distributed puncturing. It is further known from WO 01/26273A1 and WO 01/39421 A1 that it is advantageous to puncture the individual data blocks of the data stream for adapting the data rate in accordance with a specific puncturing pattern, in which case the puncturing pattern is designed in such a way that it features a puncturing rate that increases constantly from a middle area of the individual data block to at least one end of the individual data block.
An object of the present invention is, thus, to specify a method for adapting the data rate of a data stream in a communications device as well as a corresponding communications device which guarantees a satisfactory bit error rate and can be used, in particular, in mobile radio systems with convolutional coding.